A Thermodynamic Modelling Method for Organic Solid Precipitation

Abstract

Abstract A gas and oil system contains a certain amount of heavy organic substances, such as wax, resin and asphaltene which precipitate as a solid phase, causing serious problems to production for oil and gas fields, when thermodynamic conditions are changed. In this paper, a thermodynamic model of the gas-liquid- solid three-phase equilibrium for an oil and gas hydrocarbon system is presented, based on the regular solution theory, an equation of state and the phase equilibrium principle of fluid thermodynamics. A flash calculation model for the gas, liquid and solid three phases is developed, being combined with a material conservative equation. An equation of state is used to describe both gas and liquid phases, and a regular solution theory is only used to account for the non-ideality of the solid mixture, which can reflect the effects of temperature, pressure and composition of system on solid precipitation. The influence of the heat capacity difference between the liquid and the solid on the solid precipitation is taken into consideration. Some adjustable parameters and a tuning function are introduced in this model because of uncertainty of heavy component properties. It is shown that the model has a good convergency and stability, and the calculated values by this model are in good agreement with the experimental data.